docs: site preferences

[reidbarber]

Adds a dialog to adjust preferred color scheme (system, light, dark), as well as preferred package manager, styling solution, and css theme. These all persist via localstorage.

Open to feedback on the UI. I'm not completely sold on putting this button in the footer, but couldn't think of a better place. I like having in in the menu next to "Copy for LLM", but that isn't on every page. Also open to feedback on the dialog content itself.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Open a docs page and press the Settings button in the footer. Try adjusting the settings.

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

Nice! Putting the whole app into the colorscheme that isn't your OS colorscheme causes the search overlay to be wrong

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs